The present invention relates to a transistor transistor logic (TTL) driver circuit capable of producing a down-level and particularly to a TTL driver circuit that operates efficiently without the use of Schottky diodes.
A saturated state of conduction occurs when the collector voltage of a transistor reaches a sufficiently low level relative to the transistor base. When current is applied to the base of a conventional transistor, the voltage differential between the base to the emitter is 0.7 or 0.8 volts. A collector to emitter voltage drop of approximately 0.8 volts generally indicates the onset of saturation; 0.4 volts generally indicates saturation; and 0.2 volts is conventionally known as deep saturation. When saturation occurs, the gain of the transistor decreases. That is, a greater amount of current must be applied to the base in order to allow a predetermined relatively high current to flow from the collector to the emitter. Moreover, if a transistor is driven into saturation--and especially into deep saturation--when current is no longer applied to the base of the transistor, the time constant thereof changes so that the device does not shut off rapidly. The rate at which the transistor is shut off is called its switching speed. Decreased switching speeds of circuit devices necessarily result in circuit propagation delays. Such delays cannot be tolerated in many high speed electronic applications.
Saturation occurs when no clamp exists in the circuit between the base and collector of a transistor. A 0.3 volt Schottky diode clamp connected across the base-collector junctions allows current to flow through the diode, stabilizing the transistor so that, in the case of a 0.8 volt base to ground transistor for example, the collector is held substantially constant at approximately 0.5 volts and saturation is prevented.
Although Schottky diode clamps can prevent transistors from being driven into saturation over a relatively normal temperature range, when extreme temperature ranges are encountered, even Schottky devices may not perform adequately. In certain operating environments, integrated circuit chips that contain these driver circuits may be subjected to a range of temperature of 100.degree. C. or more. Such extreme temperature ranges can occur due to external factors, the heat generated by circuit components themselves, or a combination of both. Moreover, when a Schottky device is not available, temperature compensation requirements are exacerbated.
Heretofore a so-called Baker clamp circuit has been used in conjunction with transistors in order to stabilize or prevent saturation thereof. Such a clamp circuit comprises two diodes in parallel, both anodes being connected to one another and therefore having the same voltage potential. One cathode is connected to the base of the transistor, the other cathode being connected to its collector. Both of these diodes have the same voltage drop characteristics, so that the base and collector are constantly held at the same voltage potential. Extreme temperature variance during operation, however, can be a significant problem, not addressed by Baker clamps.
U.S. Pat. No. 4,533,839 issued to Balakrishnan discloses a switching output transistor in a peripheral driver circuit. The transistor is provided with a shut off circuit which turns the output transistor off when its collector supply current exceeds its saturation current. Thus, the transistor is shut off upon saturation, but no attempt is made to prevent saturation.
U.S. Pat. No. 4,277,703 issued to Wada et al. teaches the use of a level clamp circuit in conjunction with a positive feedback circuit. The level clamp circuit is coupled to the collector of a transistor in order to hold the collector output potential of the transistor higher than the base input potential thereof. Such a circuit, however, makes no allowance for temperature compensation.
U.S. Pat. No. 4,376,900 issued to Metzger teaches the use of a bipolar transistor logic circuit requiring the use of ten transistors. Certain of these transistors are prevented from being driven into saturation, but a temperature compensation mechanism is not incorporated.
U.S. Pat. No. 3,999,080 issued to Weathersby, Jr. et al. teaches the use of TTL logic with PNP input transistors to reduce loading on input drive devices. Internal clamping with p-n diode junctions prevent transistor saturation. Once again, however, temperature compensation is not addressed.
It would be advantageous to provide a circuit capable of compensating for extreme variations in temperature.
It would be advantageous to provide a non-saturating driver that would emulate a Schottky device clamp.
It would also be advantageous to provide a driver circuit in which the collector voltage of the driving transistor is kept just below the base voltage to prevent deep saturation thereof.
It would also be advantageous to provide a driver circuit in which the collector down-level voltage of the driving transistor could be adjusted by adjusting a band gap circuit voltage.
It would be advantageous to compensate for temperature in a driver circuit without using Schottky devices over very great ranges of temperature (e.g., -10.degree. C. to 100.degree. C.